7. Software and Other Intangible Assets

At March 31, 2016, software net book value totaling $208.8 million, net of $162.5 million of accumulated amortization, includes the net book value of software marketed for external sale of $56.1 million. The remaining software net book value of $152.7 million is comprised of various software that has been acquired or developed for internal use.

At December 31, 2015, software net book value totaled $237.9 million, net of $158.9 million of accumulated amortization. Included in this amount is software marketed for external sale of $70.1 million. The remaining software net book value of $167.8 million is comprised of various software that has been acquired or developed for internal use.

Quarterly amortization of software marketed for external sale is computed using the greater of the ratio of current revenues to total estimated revenues expected to be derived from the software or the straight-line method over an estimated useful life of three to ten years. Software for resale amortization expense recorded in the three months ended March 31, 2016 and 2015 totaled $3.3 million and $3.9 million, respectively. These software amortization expense amounts are reflected in cost of software license fees in the condensed consolidated statements of operations.

Quarterly amortization of software for internal use is computed using the straight-line method over an estimated useful life of three to ten years. Software for internal use includes software acquired through acquisitions that is used to provide certain of our hosted offerings. Amortization of software for internal use of $11.9 million and $8.5 million for the three months ended March 31, 2016 and 2015, respectively, is included in depreciation and amortization in the condensed consolidated statements of operations.

The carrying amount and accumulated amortization of the Company’s other intangible assets that were subject to amortization at each balance sheet date are as follows:


	March 31, 2016							December 31, 2015
(in thousands)	Gross Carrying Amount		Accumulated Amortization			Net Balance		Gross Carrying Amount		Accumulated Amortization			Net Balance
Customer relationships	$	304,913	$	(85,170	)	$	219,743	$	336,075	$	(86,585	)	$	249,490
Trademarks and tradenames		16,664		(10,122	)		6,542		18,040		(10,605	)		7,435
Purchased Contracts		10,646		(10,646	)		—		10,690		(10,690	)		—

	$	332,223	$	(105,938	)	$	226,285	$	364,805	$	(107,880	)	$	256,925

Other intangible assets amortization expense for the three months ended March 31, 2016 and 2015 totaled $5.8 million.

Based on capitalized software and other intangible assets at March 31, 2016, estimated amortization expense for future fiscal years is as follows:


Fiscal Year Ending December 31,	Software Amortization		Other Intangible Assets Amortization
(in thousands)
Remainder of 2016	$	42,207	$	15,662
2017		49,622		19,629
2018		37,313		19,122
2019		29,388		18,543
2020		24,847		17,632
2021		16,607		17,142
Thereafter		8,853		118,555

Total	$	208,837	$	226,285